Explanation:
The FTG_USE_SYS_MEMORY setting in NOTES.INI was introduced in Domino 5.0.9 to reduce memory fragmentation. When performing large full-text searches, large blocks in Notes memory can be allocated. The memory is required for a small period of time, but the Notes Memory Manager does not free the allocated memory. This results in memory fragmentation.

Setting FTG_USE_SYS_MEMORY=1 will instruct the full-text (FT) engine to directly use the operating system memory manager for large allocations, instead of requesting memory through the Notes Memory Manager.

Default:
When absent or set to 0, the full-text engine will allocate memory from the Notes Memory Manager.

Recommendation:
Disable this setting for all versions of Domino from 6.0 through 6.5.3.
Enable this setting for Domino 6.5.4 and later.

Restrictions:
Using this setting in Domino 6.0 thru 6.5.3 is likely to cause the server to crash. This issue was addressed in Domino 6.5.4. It is safe to enable this setting for Domino 6.5.4 and later.
This parameter should not be used with Domino 8.x or 8.5.x on IBM i, see Technote 1592384 for further details.

Restarts:
A server restart is required for these changes to take effect.

Related Settings:
FTG_DOMAIN_SORT_TOTAL_LIMIT, FTG_SET_INITIAL_BUFLEN, FTG_DOMAIN_SORT_TOTAL_LIMIT

References:
Technote 1116626
The Use of FTG_USE_SYS_MEMORY Causes a Crash in Domino 6
http://www.ibm.com/support/docview.wss?uid=swg21116626

Technote 1090296
How to force the full text engine to use system memory instead of Notes managed memory
http://www.ibm.com/support/docview.wss?uid=swg21090296

Release notes 7007054
Fix List for Lotus Notes and Lotus Domino Release 6.5.5 Maintenance Release (MR)
http://www.ibm.com/support/docview.wss?uid=swg27007054

Domino Configuration Tuner:
This setting is included in the DCT catalog of best practices.